Vanilla sauce: Mix sugar, vanilla sugar, cornstarch, egg yolk and milk in a saucepan.
Cook over the heat and stir until the mixture has thickened. Then add the butter.
Cover with cling film and let cool.
Sponge cake: Whisk eggs with sugar. Then add sour cream, oil and sifted dry ingredients: flour, cocoa powder, baking powder and baking soda.

Cook at 180° for 40 minutes.
Cut the cake into two layers and fill with cream.
Ganache: Melt the chocolate in the hot cream and stir well. Cover the cake with ganache.
Leave it in the fridge overnight before enjoying.